Make pg_basebackup -x stream the default

Per some discussions with a number of different people at pgconfeu, here is a patch that changes the default mode of pg_basebackup to be streaming the wal, as this is what most users would want -- and those that don't want it have to make other changes as well. Doing the "most safe" thing by default is a good idea.

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 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/